According to Wikipedia, The tragedy of the commons is an economics theory by Garrett Hardin, according to which individuals behave contrary to the whole group's long-term best interests by depleting some common resource.

The original example was common grazing land - all the commoners could buy more cattle to graze there, so they all did, resulting in over-grazing resulting in the cattle dying.

I think it now applies to roadspace - anyone can own a car, so too many do. People are reluctant to drive somewhere, if they fear it will be difficult to find a parking space there. Motorists even object to those that use less space, fearing they move more slowly.